Spider silk is a natural fiber with special physical and chemical characteristics. It has excellent tensile strength, elasticity and is resistant to mold. Spider silk gene has been demonstrated to be expressed in Escherichia coli or yeast, however, products with the mechanic characteristics of the natural fibre failed to be obtained, because the process and mechanism by which the natural silk fibre is formed from protein is still unknown. Bombyx mori has been used as a bio-reactor to commercially produce a number of polypeptides or proteins. However, the potentiality of Bombyx mori to be used as the host of expression of interesting products is very much limited for the following reasons: (1) the transgenic system of Bombyx mori is unstable and the integrated extraneous DNAs are of transient expression; (2) the mechanisms of synthesis, processing, transporting, and secretion of the silk fibroin is unknown. As a result, the application of Bombyx mori as the host for expressing foreign proteins is limited.